bouncebuf: remove dummy implementation

If any driver ever needs to use the bounce buffer API, it always needs
to use it. As such, providing a dummy implementation of those APIs when
CONFIG_BOUNCE_BUFFER isn't defined does not make sense. Remove the dummy
implementation.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Andy Fleming <afleming@freescale.com>